Communication system
BACKGROUND OF THE INVENTION 
     The invention relates to a data exchange network and more particularly to a communication system of data exchange network suitable applicable to one-way transmission of large capacity document data and image data. 
     There are two types of data exchange network, a packet switched network and a circuit switched network. In the packet switched network, since charges are calculated according to the number of packets used, this network is advantageous when the transmission data occurs only occasionally. On the other hand, in the circuit switched network, charging is made according to the time that the circuit is appropriated, and therefore this network is not advantageous for sporadic data transmission. However, the circuit switched network is capable of a transparent transmission and therefore does not require such complicated controls as required by the packet switched network in sending a large volume of data, permitting a rapid and highly efficient data transmission. 
     To make most of the features of the above two types of communication network, a communication system was devised as disclosed by the Japanese Patent Laid-Open No. 195040/1986. This is shown in FIG. 2. In this example, when a large volume of data is to be transmitted from the packet terminal 21b, a call setup packet sets a flag requesting the use of the circuit switched network. Equipment 27 attached to the packet concentrator 25 identifies this flag to secure a path through the circuit switched network, that is, through the circuit concentrators 23, 23&#39; and circuit switches 24, 24&#39; to an add-on equipment 27&#39;, a packet concentrator 25&#39; and a terminal 21b on the receiving side. When the circuit switched network appropriation request flag is not set, the packet transmission is performed through a packet switches 26, 26&#39;. 
     The above conventional communication technique is effective for conventional data transmission, for example, between TSS terminals and computer and for copying of the contents of magnetic tapes. This conventional system, however, does not include any provision for new types of data transmission that will emerge as high speed, wide band communication network advances. 
     For example, search of data base is generally performed by a computer that manages the data base. However, as the storage devices such as semiconductor memories and magnetic disks become less expensive, it is more efficient to transmit a large capacity of data to the user side where the search is done by the work station on the user side because this enables diversified processing of data. 
     As so-called electronic publishing which uses the high-speed wide-band network has come into practical use, there are growing demands for a technique that can transfer several pages of document data or image data without giving an operator an impression of system action delay with respect to the user command. 
     As mentioned above, in the high-speed wide-band network, it is necessary not only to send a large volume of information in a short period of time but also to transmit it intermittently in response to the user command. While it is possible to cope with the above requirements by using the circuit switched network, it is not economical because the large-capacity high-speed communication line is used only sporadically. On the other hand, the packet switched network has a certain limit to the increased capacity and speed because of the complexity of its control. 
     SUMMARY OF THE INVENTION 
     The above object can be achieved by increasing the speed of path setting or routing in the circuit switched network to such an extent that the system can cope with the user&#39;s demand for a large capacity data transfer. 
     In conventional analog telephone networks, the setting of path or routing has been performed by translating the dial pulses into dial information. In such networks, it necessarily takes more than 10 seconds from a call request to the initiation of communication. However, if in the trunk system the signal is transferred through the common channel signaling network and if in the subscriber line the signaling transfer using the common signaling channel of the ISDN interface becomes possible, the time required for the path setting is expected to be approximately one second. This means that as long as there is no halt to the communication line, the user demands can be taken care of with the delay of only about one second even with a type of communication network in which a call request is made every time the user request occurs. 
     Considering these facts, the above objective is achievable if the probability of the communication line being blocked is reduced and a quicker path setting is obtained. For this purpose, the invention provides an intermittent large capacity data transfer mode (simply referred to as a transfer mode) to the concentrators, switches and terminals used in the conventional circuit switched network. 
     What constitutes a large percentage of the connection delay time in the path setting which elapses from the moment of a connection demand to a path establishment is a time to establish the subscriber link. This includes power-up times of the originating and terminating network terminations and a time for establishing the frame synchronization of the subscriber link, and the subscriber link setup time is several hundred milliseconds in total. To reduce the link establishment time, the invention maintains, after data transfer, the frame synchronization of the subscriber link which consists of a terminal device, a network termination and a subscriber line terminal. That is, the subscriber link that has entered the transfer mode in response to the user demand maintains the frame synchronization unless the transfer mode is reset. This provision enables the path setting in about several hundred milliseconds. 
     On the other hand, to reduce the probability of channels being blocked, a channel of the switch, once it is set in the transfer mode, will thereafter be used only in that mode and commonly used by a plurality of transfer mode calls. When the number of calls for that mode exceeds the value determined from the traffic, a new channel is assigned for that mode. The amount of data such as mentioned before that is transferred at one time is almost equal to the capacity of the storage medium and about one hundred M bit at the most. The transfer of this amount of data can be done in less than one second with the high-speed wide-band network. That is, the plurality of calls assigned to one channel will hold the circuit for about one second only and the channel thereafter can immediately transfer other calls. A call that has accessed the channel that is handling other calls is put in a waiting queue of the right of transmission so that it can be transferred immediately after the transfer of the other calls. The length of the waiting queue can be adjusted by the user changing the number of channels assigned to the transfer mode to an allowable extent. 
     In this way the probability of the channel block can be reduced thereby achieving the above objective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is one embodiment of the communication system according to the invention; 
     FIG. 2 is an example of a conventional communication system; 
     FIG. 3 is a diagram showing the communication sequence for the system of FIG. 1; 
     FIG. 4 is a functional block diagram of the system of FIG. 1; 
     FIG. 5 is a diagram showing the equipment control flow for the system of FIG. 1; and 
     FIG. 6 is a diagram showing the communication sequence for another system according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is one embodiment of the invention and FIG. 3 shows one example of a call coupling sequence. In FIG. 1, reference numerals 11, 11&#39; represent terminal devices, 12, 12&#39; network terminations, 13 13&#39; concentrators, and 14, 14&#39; switches. Reference numerals 15, 15&#39; and 16, 16&#39; are attached devices for intermittent large capacity data transmission. 
     In this embodiment, we will explain a case where a large amount of data is intermittently transmitted from one terminal device 11&#39; to another 11 by the control from the terminal device 11. When a call request is issued from the terminal device 11, the network termination 12 sends the call request to the circuit termination of the concentrator 13. Upon reception of the call request, the concentrator 13 sends a synchronization pattern to the network termination to establish a frame synchronization with the subscriber link. After the subscriber link is synchronized, the network termination 12 sends a destination number and a communication mode identification signal to the network through a subscriber line signaling channel (D channel). 
     The path selection is made by the concentrator 13, switches 14, 14&#39;, concentrator 13&#39; on the receiving side and the destination number. At the same time, the selected channel is registered as the transfer mode in each of the added equipment 15, 15&#39;, 16, 16&#39;. In selecting the path, when there is an already registered channel in the desired direction, the call request is put in a waiting queue for the registered channel. When the waiting queue is longer than a specified value, a new channel is assigned. 
     After the path to the network termination 12&#39; on the receiving side has been selected, the concentrator 13&#39; on the receiving side activates the network termination 12&#39; which then turns its power on. After the subscriber link has been synchronized by the synchronization pattern from the concentrator 13&#39;, the concentrator 13&#39; returns a response signal to the terminal device 11 on the control side. 
     The terminal device 11 on the control side sends a data request signal to the terminal device 11&#39; on the transmission side which then transfers data. After the data is transferred, the control terminal device 11 sends out a hold signal, and the concentrators 13, 13&#39; and switches 14, 14&#39; put their right of use of the terminal devices at the end of the waiting queue. 
     When the control terminal device 11 requests the data transmission again, it sends a destination number, making the path selection by the concentrators 13, 13&#39; and switches 14, 14&#39;. Upon receiving a path selection completion signal from the transmitting terminal device, the control terminal device causes the sending terminal device to resume the transmission of data. The above sequence is repeated until the call disconnect request is made from the terminal device 11. 
     FIG. 4 shows the functional block diagram of the concentrators 13, 13&#39;. In the Figure, reference numerals 41, 41&#39; are subscriber line terminals, 42 a subscriber line interface, 43 a concentrator, 44 a scanner, 45 a trunk scanner, and 46 to 49 added equipment associated with transfer mode. 
     A call request from a subscriber is accepted by the scanner 44 which is then connected to an idle concentrator. When the call demands a transfer mode, it is registered in a queue by an intermittent transfer mode control 47. An intermittent transfer mode scanner 46, according to the waiting queue, performs interlaced-scanning on the subscriber line interface 42 and, when there is a transfer request, accepts it and sets a path. After the data transfer has been completed and the control terminal device 11 has sent a hold signal, the intermittent transfer mode control 47 puts the right of link use by the terminal device at the end of the waiting queue. 
     An incoming call, on the other hand, is identified by a trunk scanner 45 and the path to the subscriber line terminal 41 is set by controlling the concentrator. The subscriber line terminal 41 activates the network termination 12&#39; to synchronize the subscriber link. When the incoming call demands the transfer mode, the intermittent transfer mode trunk control 49 identifies a trunk being used in the transfer mode and puts the call at the end of the waiting queue for the transfer mode link. The intermittent transfer mode trunk scanner 48, according to the waiting queue, performs the interlaced-scanning on the trunk to accept the transfer request. According to the request, the call processor 40 controls the concentrator 43 to set a path. 
     FIG. 5 shows a control flow of this embodiment. The communication mode is identified from the subscriber signal transmitted to the control system. When a path is to be set according to the destination number, the reference to the link administration table and trunk administration table is controlled from the result of mode identification to set a path for the transfer mode. This control is executed in response to the call until the call is disconnected. 
     As explained above, the invention permits the execution of the transfer mode only by adding the add-on equipment to the existing concentrators and switches in the circuit switched network. Another advantage is that priority control items can be added for desired control with only small changes in software. This facilitates introduction of the system of the invention into the existing communication network. 
     FIG. 6 shows another embodiment of the invention. In this example, the information of the path established at the time of making a call, i.e., the channel control information, is stored in the added equipment. When the path is to be reestablished, the information is read out to control the channel. When the channel is busy, the system waits until the channel is released. That is, the channel is established in a CSMA/CD (carrier sense multiple access/collision detection) procedure. 
     According to this embodiment, since the channel can be established with a simple control means, the control carried out in each of the network equipment is simple. 
     Also for a two-way large volume data transfer as opposed to the one-way data transfer described above, the method of this invention can also be suitably applied. 
     As mentioned above, this invention achieves an increased speed in setting the path by using the common line signal network and the outband signal channels of the subscriber line. 
     Furthermore, the intermittent large volume data transfer mode is provided to each network equipment so that when the equipment is to be operated in that mode, the frame synchronization of the subscriber line is maintained during the hold state, thereby increasing the speed of reestablishing the path. 
     The above two features permit the intermittent large volume data transfer through the common use of each network equipment. This ensures the effective utilization of the network equipment, providing more economical communication services.